Optical surfaces of eyeglass lenses are typically prepared in accordance with particular lens prescriptions that require the lens have that contours which provide selected focusing or other optical effects. The contours may be convex or concave, and a lens may be provided with both convex and concave surfaces that act together to produce the desired optical effect. Generally, the surface of an eyeglass lens proximate to the eye, is ground with a concave contour made up of compound curves. A toric surface is found on many ophthalmic lenses, and has the contours of a section of a toroid or donut. In general, there are two basic curvatures on a toric surface, one corresponding to the radius of the equator and the other corresponding to the radius of the tubular element forming the toroid. These two curvatures are referred to respectively as the “sphere” and the “cylinder”, and together with the “axis” angle of the cylinder, the spherical curvature of the front surface and the index of refraction of the lens material define the prescription power of the lens.
Conventional methods for finishing the prescription surface of an ophthalmic lens utilize a lap having a specially contoured surface that substantially matches the contours of the desired prescription surface of a lens being finished. A thin finishing pad is attached to the contoured surface of the lap, typically by adhesive, and an abrasive material is either directed onto the pad in the form of a slurry or is incorporated into the pad itself. Typically, pads with an abrasive material bonded or otherwise integrated into them are referred to as fining pads and are used for coarser finishing operations. Fibrous pads without abrasives are used with a slurry containing fine abrasive materials are referred too as polishing pads. Unless otherwise specifically stated, the term “finishing pad” is utilized throughout this specification to refer to both types of pads, and the term “finishing” is used throughout this specification to refer to both types of operations.
Since the finishing pad in conventional finishing operations is relatively thin and must take its shape from the lap, the lap in turn must be ground with contours that essentially conform to the prescription or curvatures of the lens being finished. As a consequence, finishing laboratories must stock a large number of laps corresponding to the full range of prescriptions that are commonly required. Needless to say, a significant inventory of laps is needed.
Alternatively, an individual lap can be ground for each prescription as needed. U.S. Pat. No. 4,989,316 issued to Logan et al. and assigned to the Assignee of the present invention, describes a numerically-controlled machine for cutting a lens blank and a corresponding lap blank to be used in finishing the lens blank.
As a further alternative, the lens blank from which the eyeglass lens is formed can be coarsely ground to the desired prescription, and a conformable lap can be used as the tool for the finishing operation. A conformable lap in general has a work surface that is adapted to conform to the curvature of the contoured surface ground on the lens blank. Thus, during a finishing operation which may employ a fining or polishing pad with slurry, the coarseness of the contoured surface is removed but the general curvatures defined by the prescription are preserved. Conformable laps are shown in U.S. Pat. Nos. 2,654,027; 4,831,789; 5,095,660; 5,345,725; and 5,593,340, as well as European Application No. 0 655 297.
It is an object of the present invention to provide conformable laps which may conform to the contours of optical surfaces having a wide range of curvatures, and a related method for finishing such optical surfaces.